It's rare to find fossils of animals doing anything other than, well, dying. Sure, occasionally we'll find a brooding mother with her eggs, or animals locked in mortal combat — but the vast majority of fossilized remains are simply in the midst of their death throes.

But now, for the first time, we have fossil evidence of ancient vertebrates getting it on.

In fact, we've found not just one pair, but a number of them, preserved in the Eocene Messel Lake Fossil Pit in Germany. Famed for the incredibly preserved fossils found therein, this 47 million year old pit has yielded more than 50 specimens of the turtle Allaeochelys crassesculpta — of which 18 are found in pairs.

Now, for the first time, these pairs have been been positively identified as male/female sets, and it really looks like they died while making the beast with two shells. This research, presented in the Royal Society's Journal Biology Letters, identified the turtles' sex by the fact that the males have longer tails and smaller bodies, while the females have "plastral kinesis along the hyoplastral/xiphiplastral suture".

Seven of the nine pairs are in direct contact in the fossil record — but the most telling sign is in two of them, where "the tails of the male individuals wrap below the carapace of the female and are aligned with those of the female." This, evidently, is important for turtle mating.

So how did these specimens of A. crassesculpta experience a fatal case of coitus interruptus ? Unfortunately, it's not known what about Messel Lake made it so deadly to so many animals, but the researchers think it may have have to do with layers of toxic water. They think that as the turtles mated, they sunk lower into the lake itself, and these deeper levels may have been rendered poisonous by a build up of volcanic gases or decay of organic matter. They would have died on the way down, and been preserved on the bottom.